1. Field of the Invention
The present invention relates to a method and apparatus for de-mapping symbols, and more particularly to a method and apparatus for de-mapping symbols in a Multi-Input Multi-Output (MIMO) communication system.
2. Description of Related Art
As people's requirements for the wireless communication services have become increasingly high, the current wireless communication system has a trend of having a high data rate and a high link quality. In order to effectively increase the data transmission rate and the link quality under limited bandwidth, a wireless communication system using a plurality of antennae is developed, for example, Multi-Input Multi-Output (MIMO) communication system (briefly referred to as MIMO system). Both the transmitting end and receiving end of the MIMO system utilize a plurality of antennae for data transmission, which not only enables the data to be transmitted in multiple paths and increases the data transmission volume, but also prolongs the transmission distance for the data, and thus the MIMO system has become one of the main wireless communication systems nowadays. The Bell-labs Layered Space Time (BLAST) communication technique developed by Bell labs of Lucent Technologies in USA is the most representative one.
In the MIMO system, the data received by the receiving end is the data formed by mixing the data transmitted from the transmitting end via the channel effect. For example, FIG. 1 is a block diagram of a conventional 4×4 MIMO communication system. Referring to FIG. 1, the transmitting end of the communication system 100 includes an encoder 101, a demultiplex (DEMUX) 102, mappers 103-106, waveform shapers 107-110 and transmitting antennas 111-114. The receiving end includes receiving antennas 115-118, match filters 119-122, a detector 123 and a decoder 124.
In the transmitting end, the encoder 101 is used to encode a data bit stream am into a serial bit stream bn. The DEMUX 102 is used to decompose the serial bit stream bn into a plurality of sub bit streams for being transmitted respectively via the transmitting antennas 111-114 subsequently. The mappers 103-106 are used to modulate each of the sub bit streams into a data symbol xk0, xk1, xk2, xk3, and then, after the signals are respectively shaped via the corresponding waveform shapers 107-110, the signals are transmitted to the receiving end via the corresponding transmitting antennas 111-114 respectively.
In the other aspect, in the receiving end, the receiving antennas 115-118 are used to receive data symbols transmitted via wireless channels. The match filters 119-122 are used to distinguish the signals into yk0, yk1, yk2, yk3. The detector 123 is used to estimate the data symbols originally transmitted via the transmitting antennas 111-114 according to the received data symbols, and estimate the serial bit stream {circumflex over (b)}n accordingly. The decoder 124 is used to decode the estimated serial bit stream {circumflex over (b)}n back to the data bit stream âm. After the data symbols transmitted via the transmitting antennas 111-114 having been estimated, but before being decoded by the decoder 124, a soft value must be calculated for the decoder 124 in order to perform the better decoding process.
To sum up, since the data symbols (yk0, yk1, yk2, yk3) received by the receiving antennas 115-118 are results formed by mixing all the transmitted data symbols (xk0, xk1, xk2, xk3) via the channel effect, the receiving end must estimate the data symbols transmitted from each of the transmitting antennas 111-114 of the transmitting end from the received data symbols, thereby de-mapping the received data symbols, in this way, the receiving end can perform the subsequent data calculation processes.
In the conventional detection mechanism, Maximum Likelihood (ML) algorithm is an ideal mechanism. However, the ML algorithm must search all the possibilities, such that the calculation is excessively complicated. Taking the 4×4 (the number of the receiving antennas×the number of the transmitting antennas), 16 Quadrature Amplitude Modulation (QAM) system for an example, there are totally (164) possibilities, such that the ML algorithm is actually difficult to be achieved in the actual application. In order to cater to the actual application, other detection mechanisms are developed, such as Zero Forcing (ZF) algorithm, Minimum Mean Square Error (MMSE) algorithm, and Vertical BLAST (V-BLAST) algorithm.
The ZF algorithm and the MMSE algorithm belong to linear detection methods, which mainly aim at finding out an inverse channel for the channel effect, and then estimate the data symbols transmitted via each transmitting antenna by eliminating the channel effect merely through simple calculation. The V-BLAST algorithm belongs to an iterative detection method, which finds out a nulling vector, multiplies the nulling vector by the received signals as the estimated result, and then, removes the effects caused by the estimated result from the received signals to serve as the new received signals. Such calculations are repeated in several steps, until all the results are estimated.
In view of the above, although the V-BLAST algorithm has a better performance than that of the ZF algorithm or the MMSE algorithm, it has more complicated calculations, especially inverse matrix calculation must be performed during the calculations, and it must be calculated for each iteration. When the number of the antenna is increased, the required calculation complexity is also increased accordingly. Therefore, how to solve the problem of the calculation complexity in de-mapping symbols and meanwhile achieve better performance is one of the important issues of the current multi-antenna communication development.